What piece of horror art or entertainment changed the genre of horror films?
The Answer: NIGHT OF THE LIVING DEAD.
I think without question it ushered in an era of graphic horror even more than any of the gore flicks that preceded it did. It was graphic and grim and realistic. The zombie rules it established have been so widely copied in films that followed.
The plot is simple. Set in a rural Pennsylvainia farmhouse and battling constant attacks from dead locals who have been brought back to life by mysterious radiation, human nature is put to the test.
This movie is fantastic. There are still very few great horror films out there, simply because they do not display true human nature under terrifying circumstances. Night of the Living Dead is a beautifully created film that has earned its place in both film and horror history.